1. Field of the Invention
This invention relates to a synchronizing clock generating circuit for generating clock signals synchronized with e.g. synchronizing signals for video signals.
2. Description of the Prior Art
Among synchronizing clock generating circuits for generating clocks timed to synchronizing signals of video signals, that is synchronizing clock signals, there is, for example, a clock generating circuit for clock signals for writing in a memory arranged in a time base corrector (TBC) which is adapted for correcting the time base for video signals (synchronizing clock signals). This synchronizing clock generating circuit is arranged as shown for example in FIG. 1.
In this figure, playback video signals from e.g. a video tape recorder (VTR) are supplied to an input terminal 10 and thence supplied to a synchronizing signal separating circuit 11, where horizontal synchronization signals SH are separated from playback video signals also containing vertical synchronization signals. The separated horizontal synchronization signals SH are supplied to a reference pulse generating circuit 12, where reference pulses RP are generated on the basis of the horizontal synchronization signals and transmitted to a phase comparator 53 which will be described subsequently.
The reference pulses RP from the reference pulse generator 12 are also supplied to a voltage-controlled oscillator (VCO) 15. Write clock signals WCK to be transmitted to the above-mentioned TBC memory, that is, synchronizing clock signals, are generated by VCO 15 so as to be supplied via a terminal 17 to the TBC. The VCO 15 is controlled on the basis of the results of phase comparison of the synchronizing clock signals and the reference pulses RP generated from the horizontal synchronization signals contained in the video signals. That is, the results of phase comparison are fed back to the VCO 15 via a lowpass filter (LPF) 14 by a phase locked loop (PLL) for phase matching the synchronizing clock signals to the reference pulses RP derived from the horizontal synchronization signals contained in the video signals.
To this end, the above-mentioned synchronizing clock signals, outputted from VCO 15, are supplied to a PLL counter 16 which counts the number of clocks of the synchronizing clock signals to transmit the results to a count H generator 58. This count H generator generates a trigger pulse CH having a frequency equal to a submultiple N of the frequency of the write clocks (synchronizing clock signal) WCK, where N may be 858 or 910 etc.
This trigger pulse CH is sent to a trapezoidal wave generator 59, where a trapezoidal wave is generated from the falling edge of the trigger pulse CH. The trapezoidal wave is transmitted to a phase comparator 53. It is this phase comparator that is used for effecting phase comparison between the above-mentioned reference pulse RP and the synchronizing clock signals. In the phase comparator 53, the above-mentioned synchronizing clock signal and the reference pulse RP are phase-compared to each other with the aid of the trapezoidal wave.
Specifically, the trapezoidal wave is sampled in the phase comparator 53 with teh reference pulses RP. An output of the phase comparator 53 is fed back as error signals to VCO 15 via the above-mentioned lowpass filter 14 for controlling the VCO 15.
The above-mentioned error signals from the phase comparator 53 are transmitted to the lowpass filter 14 via a sample-and-hold circuit 56.
Meanwhile, the falling edge position of the above-mentioned trigger pulse CH from the count H generator 58 is produced at a position N-n ahead of N in terms of a decoding value of the PLL counter 16, where N is a number, such as 910, corresponding to the number of clock pulses in one horizontal line (1H) of a video signal and n may for example be 14. Thus the trigger pulse CH falls at a position N-n or 910-14=896 ahead of N=910. Thus the trapezoidal wave generator 59 generates the trapezoidal wave sloped down from this falling position of the trigger pulse CH. At a timing of the next 1H, corresponding to n counts, herein 14 counts, from this falling position of the trigger pulse CH, a reference pulse RP is generated by the reference pulse generator 12 for phase comparison in the phase comparator 53.
If the next reference pulse RP is delayed, it samples the latter half of the trapezoidal wave, whereas, if the next reference pulse RP arrives at a premature time, it samples the former half of the trapezoidal wave. In both of these cases, error signals are supplied via sample-and-hold circuit 16 and LPF 14 to VCO 15 so that the clock WCK generated by VCO 15 are synchronized with the reference pulse RP.
However, it may occur that the gradient of the descending slope of the trapezoidal wave from the trapezoidal wave generator 59 be changed with lapse of time or due to fluctuations from circuit to circuit or fluctuations in temperature characteristics. What happens in such case is explained by referring to FIG. 2.
It is supposed that the gradient b of the trapezoidal wave is normal during reproduction at a normal tape speed. The falling of the trigger pulse CH occurs n clocks ahead of N clocks corresponding to 1H. A sampling value of the gradient b by a reference pulse RPb coming after lapse of a time t corresponding to the n clocks since the falling of the trigger pulse CH is a reference level and, if the sampling value of the gradient b is not equal to this reference level, error signals are produced by the phase comparator 53 for controlling the VCO 15.
If the gradient of the trapezoidal wave is changed by some reason and become a gradient a, the VCO 15 is locked so that the reference pulse RP is at a reference level a' having a gradient a, so that the relation of the trapezoidal wave with the reference pulse RP is as shown at RPa. Although the positions of the RP pulses are shown to be deviated in FIG. 2, the RP pulses are generated at an interval equal to that of the synchronizing pulses which is basically constant, so that, if the gradient of the trapezoidal wave becomes acute, as at a, the VCO 15 is controlled in such a manner that the falling position of the trigger pulse CH is delayed. Since the number of count clocks up to the trigger pulse CH is constant at N-n which is equal to e.g. 896, the VCO 15 has to cope with the situation by lowering the clock frequency for delaying the falling position of the trigger pulse CH. In such case, the write clocks WCK are lowered in frequency so that the TBC supplied with these WCK pulses cannot be operated normally.
If the gradient of the trapezoidal wave becomes milder as at c, the VCO 15 controls clocks so that the reference pulse RP is at a reference level c' having a gradient c. In such case, the VCO 15 has to cope with the situation by raising the clock frequency for advancing the falling position of the trigger pulse CH, so that again the TBC is unable to be operated normally.
It is noted that the length of the 1H period of the playback video signals is changed with the reproducing speed and hence the correct gradient of the trapezoidal wave is similarly changed depending on the reproducing state or with the reproducing speed. Heretofore, changes in the gradient were controlled in an open loop on the basis of the reproducing speed supplied from terminal 60.
In this case, however, the above-mentioned frequency deviations are unavoidably produced due to, for example, temperature characteristics or deterioration of various components with lapse of time, such that degradations such as bends or distortions in the reproduced images are produced by these frequency deviations, thereby lowering the picture quality.
Meanwhile, if the reference pulses should fail to be produced by signal dropout, the VCO 15 or the PLL counter 16 is not reset, so that the frequency deviations are directly manifested as bends in the reproduced image.